Semiconductor memories may include threshold-type memory cells, which are memory cells that are accessed by providing a voltage across a memory cell, and the data value stored is based on a threshold voltage of the memory cell. For example, the data value may be based on whether the threshold voltage of the memory cell is exceeded when a voltage is applied across the memory cell, and whether the memory cell conducts current in response to the voltage provided across the memory cell. The data value stored may be changed, for example, by applying a voltage sufficient to change the threshold voltage of the memory cell. An example of a threshold-type memory cell may be a phase change memory cell.
The voltage applied across a memory cell is typically provided by two access lines to which the memory cell is coupled. Also coupled to each of the access lines are other memory cells. Each of the memory cells may have a respective threshold voltage, with some having a relatively low threshold voltage and others having a relatively high threshold voltage. When a voltage is applied to a target memory cell by the two access lines, the other memory cells coupled to the respective access lines are also subjected a respective voltage. While the respective voltage to which the other memory cells is subject is not sufficient to access the other memory cells also coupled to the respective lines, the voltage may nonetheless be sufficiently high to inadvertently degrade or change the threshold voltage of some of the other cells. For example, where one of the other memory cells has a relatively low threshold voltage, and the target memory cell has a relatively high threshold voltage, the voltage to which the other memory cell is subjected when accessing the target memory cell may be sufficient to degrade the relatively low threshold voltage (e.g., increase the relatively low threshold voltage), or change the relatively low threshold voltage to a relatively high threshold voltage, thereby changing the data value stored by the other memory cell. If such an event were to occur, the memory has failed.
Thus, there is a desire to have apparatuses and methods for accessing memory cells that mitigate application of access voltages that may degrade performance of the memory.